1. Field of the Invention
The present invention relates generally to optical tape data storage systems, and more specifically to a focal plane stabilizer that maintains the optical tape surface in the focal plane of an objective lens.
2. Related Art
The advent of optical storage technology brought about the ability to achieve greater recording densities than could be achieved using conventional magnetic recording technology. As a result, developers in the tape storage industry are turning to optical recording technology to meet their mass data storage needs.
One area of recent development is that of the optical tape drive. Such a tape drive uses an optical tape similar to magnetic tapes currently used in magnetic tape drives. Because the read and write operations are performed using electro-optics that focus a beam of light on the tape, tape alignment and stability are critical. To minimize the number of data storage and retrieval errors, it is important to maintain the area of the tape at which data is written to and/or read from (i.e., the focal plane area) in stable alignment with the focal plane of the objective lens.
One conventional approach to providing a stable focal plane is illustrated in FIGS. 1A-1C. FIG. 1A is a diagram illustrating a conventional focal plane stabilizer. Referring now to FIG. 1A, a conventional stabilizer 100 comprises a radius contour 102 over which the optical tape is positioned during read and write operations. The conventional contour 102 has a number of airholes 112 on a surface 110 of the contour. Air is forced out of these holes 112 to form an air cushion between the optical tape and the contour 102. This air cushion, known as a hydrostatic bearing, is intended to hold the tape steady during the read/write operations. In this configuration, the focal plane area is proximate to the contour apex.
FIG 1B is a diagram illustrating the relationship between the optical tape 122 and the radius contour 102. As a result of the air being forced through airholes 112, tape 122 is separated from radius contour 102 by a height "H." According to this conventional stabilizer, the separation H is a function of the number, pitch, and distribution of the airholes 112 and the airflow through them.
According to this design, the optical tape 122 is moved across the head in the direction illustrated by arrow A. The tape is stopped and data is written to the tape from edge to edge across the tape while the tape remains stationary. This is usually accomplished by scanning the objective lens across the width of the tape in a direction perpendicular to the sheet of FIG. 1B. When this write operation is complete the tape is stepped (moved the width of one or more tracks and stopped) and another write operation occurs. This process of stepping and writing continues until the tape is full or there are no more write operations to perform.
There are several disadvantages associated with this conventional design. One disadvantage is that data can not be written to the tape while it is moving. Instead, the tape is stationary during the recording process, and the lens is scanned across the width of the tape. This necessarily requires more time than would be required to record on a moving tape because of the time associated with starting and stopping the tape.
Another disadvantage, illustrated in FIG. 1C, is that a dirt particle 144 trapped between tape 122 and contour 102 can cause a perturbations in the tape surface known as a "tent" 152. If large enough, tent 152 can cause the tape to fall outside the focal plane of the objective lens. This can result in data errors. By increasing the spacing H, the impact of trapped dirt particles 144 on the write/read process can be minimized. However, as H increases so does the tape sensitivity to low frequency excitations. This can result in tape vibrations referred to as tape flutter. When the tape flutter amplitude becomes excessive write/read errors can occur.
The tape flutter problem is intensified when the tape is moving. This contributes to the impracticality of using this conventional focal plane stabilizer design in systems that write and read to moving tapes. In fact, because of tape flutter, it is virtually impossible to read and write at a desired tape speed of four meters per second (4 m/s).
Radius contour tape heads are also used in the magnetic tape industry. FIG. 2 is a diagram illustrating one conventional magnetic tape head design. Referring now to FIG. 2, a tape head contour 202A is provided to write data to a magnetic tape passing across the tape head. A write gap 222 centered on contour 202A writes data to the tape. The apex of the contour 202A and the gap 222 need not coincide. The radius of the contour 202A affects the flying height of the tape. Bleed slots 230 are often provided to further adjust the flying height.
If both read and write operations are desired, a second contour 202B is added. Contour 202B includes a read gap 224 centered thereon and used to read data from a magnetic tape passing over contour 202B. According to this design, the critical area for tape stability is in the area of the gaps of contours 202A and 202B. Two adjacent contours are used to allow both read and write operations to take place simultaneously.
This conventional magnetic tape head configuration is illustrated and described in the article "Historical Perspective of Tape Head Contours" by F. William Hahn, Jr., pg. 23, date unknown. The article, published in Tribology and Mechanics of Magnetic Storage. Systems, is incorporated herein by reference.